CONTENTS
Lear ning Objectiv
Learning
+ 0 ) 2 6 - 4
es
Objectives
"$
Ï Classes of Electronic AC
Drives
ELECTRONIC
Ï Variable Frequency Speed
Control of a SCIM
Ï Variable Voltage Speed
CONTROL OF
Control of a SCIM
Ï Chopper Speed Control of
a WRIM
A.C. MOTORS
Ï Electronic Speed Control
of Synchronous Motors
Ï Speed Control by Current-
fed D.C. Link
Ï Synchronous Motor and
Cycloconverter
Efficient control of motors becomes critical
Ç where high precision, accuracy, flexibility,
reliability and faster response are of para-
mount importance. Electronic and digital
controls are employed in such conditions
CONTENTS
1824 Electrical Technology
46.1. Classes of Electronic A.C. Drives
AC motors, particularly, the squirrel-cage and wound-rotor induction motors as well as
synchronous motors lend themselves well to electronic control of their speed and torque. Such a
control is usually exercised by varying voltage and frequency. Majority of the electronic a.c. drives
can be grouped under the following broad classes :
1. static frequency changers like cyclo-converters which convert incoming high line frequency
directly into the desired low load frequency. Cyclo-converters are used both for synchronous
and squirrel-cage induction motors.
2. variable-voltage controllers which control the speed and torque by varying the a.c. voltage
with the help of SCRs and gate turn-off thyristors (GTOs).
3. rectifier-inverter systems with natural commutation.
4. rectifier-inverter systems with self-commutation.
Fig. 46.1
46.2. Variable-frequency Speed Control of a SCIM
Fig. 46.1 shows a 3-phase SCIM connected to the outputs of three 3-phase cycloconverters. As
seen, each cyclo-converter consists of two 3-phase thyristor bridges, each fed by the same 3-phase,
50-Hz line. The +R bridge generates the positive half-cycle for R-phase whereas −R generates the
negative half. The frequency of the cycloconverter output can be reduced to any value (even upto
zero) by controlling the application of firing pulses to the thyristor gates. This low frequency permits
excellent speed control. For example, the speed of a 4-pole induction motor can be varied from zero
to 1200 rpm on a 50-Hz line by varying the output frequency of the cycloconverter from zero to
40 Hz. The stator voltage is adjusted in proportion to the frequency in order to maintain a constant
flux in the motor.
Electronic Control of A.C. Motors 1825
This arrangement provides excellent torque/speed characteristics in all 4-quadrants including
regenerative braking. However, such cycloconverter-fed motors run about 10°C hotter than normal
and hence require adequate cooling. A small part of the reactive power required by SCIM is provided
by the cycloconverter, the rest being supplied by the 3-phase line. Consequently, power factor is poor
which makes cycloconverter drives feasible only on small and medium power induction motors.
46.3. Variable Voltage Speed Control of a SCIM
In this method, the speed of a SCIM is varied by varying the stator voltage with the help of three
sets of SCRs connected back-to back (Fig. 46.2). The stator voltage is reduced by delaying the firing
(or triggering) of the thyristors. If we delay the firing pulses by 100°, the voltage obtained is about 50%
of the rated voltage which decreases the motor speed considerably.
Fig. 46.2
Unfortunately, I2R losses are considerable due to distortion in voltage. Moreover, p.f. is also
low due to large lag between the current and voltage. Hence, this electronic speed control method
is feasible for motors rated below 15 kW but is quite suitable for
small hoists which get enough time to cool off because of intermit-
tent working. Of course, p.f. can be improved by using special thy-
ristors called gate turn-off thyristors (GTOs) which force the current to
flow almost in phase with the voltage (or even lead it).
46.4. Speed Control of a SCIM with Rectifier-
Inverter System
A rectifier-inverter system with a d.c. link is used to control the
speed of a SCIM. The inverter used is a self-commuted type (differ-
ent from a naturally commutated type) which converts d.c. power A commonly used electronic
into a.c. power at a frequency determined by the frequency of the power inverter
1826 Electrical Technology
pulses applied to the thyristor gates. The rectifier is connected to the 3-phase supply line whereas the
inverter is connected